X-ray binaries consist of a compact object orbiting a normal star, i.e, a star still carrying out the conversion of hydrogen into helium through thermonuclear reactions. Based on the nature of the compact companion, X-ray binaries divide up into black-hole, neutron star and white dwarf systems. Traditionally, though, the term X-ray binaries has been used to designate the neutron star group, while systems harboring a white dwarf are known as cataclysimic variables. Those having a black hole are simply called black-hole systems. The high-energy radiation, i.e., the X-rays, are the result of the accretion mechanism. The word accretion refers to any gradual accumulation or deposition of diffuse gas or matter onto some object under the influence of gravity. In an X-ray binary systems the material that is falling on to the compact object comes from the optical and bright companion. In the case of a neutron star or a black hole the matter falls down onto an enormous well of gravitational potential and is accelerated to extremely high velocities. Assuming all the matter accumulates on (or near) the surface of the object, the kinetic energy of free-fall has to be radiated away as heat which is available to power the X-ray source. The free-fall velocities at the region where the kinetic energy is radiated away (the surface of the star or just above it) are of the order of half of the velocity of light.
X-ray binaries containing a neutron star or black hole are divided up into high mass (HMXB) and low mass X-ray binaries (LMXB) depending on the spectral type of the mass donor as this feature determines the mode of transferring mass to the compact object and the environment surrounding the X-ray source. In LMXB, the donor spectral type is later than type A and has a masssmaller than two solar masses. The HMXBs contain a type O or B companion whose luminosity is comparable or greater than that from the X-ray source and for which M > 10 solar masses. HMXBs are further divided into two groups according to the luminosity class of the optical companion: the Supergiant X-ray Binaries (SXRB), sometimes called "standard" massive X-ray binaries with luminosity classes I-II and the Be/X-ray binaries with luminosity classes III-V.
BeX consist of a neutron star orbiting a O9e-B2e main-sequence star. The letter "e" stands for emission, as instead of the normal photospheric absorption lines, the optical spectra of Be stars display emission lines. Strong infrared emission and polarized light are another two defining characteristic of Be stars. The origin of these three observational properties (emission lines, infrared excess, and optical polarization) resides in a gaseous, equatorially concentrated circumstellar disc around the OB star. This disc constitutes the main source of variability in BeX.
The matter transferred from the disc to the neutron star is responsible for the emission of high-energy radiation as a result of accretion. Thus, X-rays give information about the physical conditions in the vicinity of the compact object, while optical/IR observations provide information about the massive companion. A multiwavelength approach is then the best way to unveil the properties of this type of systems.
The staff of the Skinakas Observatory is currently involved in the following on-going projects:
The objective of this monitoring program is to study the long-term variability in order to determine the structure and distribution of the circumstellar material and to characterize the variability time scales associated with the disc. In Be/X-ray binaries, the photometric and spectroscopic variability is generally dominated by long-term changes (months to years) associated with the evolution (loss and re-formation) of the circumstellar disc, which in turn, correlates with the X-ray emission. Some of the topics addressed in this project are:
V/R variability: The study of the formation and variability of the spectral lines in Be stars is of great importance since it provides information about the geometry, density and kinematics of the circumstellar matter. In particular, the so called V/R variability has attracted the attention of many researchers. Current models suggest that V/R spectral variations seen in Balmer lines are caused by the precession of a density perturbation in the circumstellar envelope of the Be star.The density enhancement affects the shape of the Balmer emission lines whose asymmetric double-peaked profiles alternate between red (VR) emission
Interaction of the neutron star with the Be star's envelope:
Be stars may have an isolated life or take part in binaries (the BeX systems). The difference is the presence, or not, of a neutron star. Here the objective is to investigate the effects of the compact object on the structure and evolution of the circumstellar envelope. One of the most interesting effects is the truncation of this envelope by the neutron star. One of the main goals is to find observational evidence of such truncation.
-Search for optical counterparts to HMXB:
With the improved sensibilitiesofthe currently operational space missions new X-ray sources are being discovered. However, many of these sources are of unidentifiednature. An optical identification is necessary to facilitate a complete study of these systems. Without a known counterpart, observations are limited to X-ray energies, and hence our understanding of the structure and dynamics of those systems that remain optically unidentified is incomplete. If the field of view of the instrument is small enough then ground-base observations can help identify the correct counterpart. The observation of emission lines of the Balmer series and IR excess are the first indications of the presence of an aearly-type star. Optical classification is then performed to confirm the nature of the X-ray system definitively.
The circumstellar disk, also known as decretion disk, does not originate as a result of accretion from an external source but as a result of episodic mass loss from the star itself. The gas ejection mechanism is not known. However, it is believed that the rotation rate of the Be star is a key ingredient in the understanding of the formation of the equatorial disk. The higher the rotation rate the easier to lift up gas and launch material into a ballistic orbit. Non-radial pulsations, combined with rapid rotation, are the best candidates to explain mass-loss and disk-formation in hot stars.